The present invention relates generally to nuclear fuel assemblies for use in nuclear reactors. More specifically, the present invention relates to an improved nuclear fuel assembly especially adapted for use in a boiling water reactor to improve the undermoderated condition that can exist at the top of a boiling water reactor fuel assembly.
Of course, it is known to generate large amounts of heat and energy through nuclear fission in a nuclear reactor. Energy is dissipated as heat in elongated nuclear fuel rods. Typically, a nuclear fuel assembly includes a number of nuclear fuel rods that are grouped together to form a nuclear fuel assembly. Such fuel assemblies include a number of elongated rods supported between upper and lower tie plates. For example, nuclear fuel assemblies can be formed by a 7.times.7, 8.times.8, 9.times.9, 10.times.10, and 11.times.11 rod array.
In a typical boiling water reactor fuel assembly, the assembly may only include fuel rods or may also include moderation rods. For example, in a nuclear fuel assembly having a 10.times.10 array, the 100 rods that form the array may either be 100 fuel rods or they can contain 1 or more non-fuel, water moderator rods with the remaining rods being fuel rods.
In this regard, early boiling water reactor fuel designs usually employed one or two water rods near the center of the fuel assembly which displaced one or two fuel rods. The purpose of the water rods was to place more water into the center of the assembly to improve neutron moderation and uranium utilization in the surrounding fuel rods.
Some more recent designs have employed an even greater number of water rods in the fuel assembly or have used large water channels. For example, Advanced Nuclear Fuels Corporation of Richland, Wash., U.S.A., has fabricated a 9.times.9-5 design which includes 76 fuel rods and 5 water rods and a 9.times.9 which includes 72 fuel rods and an internal water channel that displaces 9 fuel rods.
One of the problems encountered with boiling water reactor nuclear fuel assemblies is the undermoderation and overenrichment of the center region of such fuel assemblies. Further, the steam volume fraction in the water coolant of a boiling water reactor increases with respect to the height of the reactor core due to the generation of steam along the length of the fuel rods in the boiling process increasing the under-moderation.
At the bottom of the boiling water reactor fuel assembly, essentially 100% unvoided water surrounds the fuel rods. But, the percentage of unvoided water decreases along the length of the fuel assembly toward the top of the assembly. Accordingly, near the bottom of the fuel assembly, sufficient water exists to provide effective neutron moderation. However, due to the presence of steam in the upper part of the assembly, the assembly may be undermoderated. The addition of water rods or water channels to the fuel assembly provides water in the upper portion of the assembly that is needed to improve neutron moderation and enhance uranium utilization efficiency.
Additional designs have been proposed to overcome some of the difficulties encountered with respect to the moderation of neutrons and thus, uranium utilization efficiency. U.S. Pat. No. 4,957,698 discloses a fuel design that preferentially directs more unvoided water coolant into the upper portion of the fuel assembly. This allows relatively more fuel to be placed in the lower portion of the fuel assembly. The arrangement is designed to allow moderation of neutrons in the upper portion of the assembly while preserving a higher volume of fuel in the lower portion. The large number of fuel rods that can be used in the lower portion reduces the linear heat generation rate (power peaking) in the assembly.
U.S. Pat. No. 4,664,882 discusses a segmented fuel and moderator rod and fuel assembly for a boiling water reactor. The segmented rod has a lower fuel region and an upper moderator region for passing coolant through the upper portion of the boiling water reactor core which is normally undermoderated. The segmented rod displaces one or more conventional fuel rods in the fuel bundle.
Fuel rods including a water rod portion are also disclosed in Japanese Laid Open Application Nos. 59-18039 and 60-201284.
German Published Patent Application (Offenlegungsschrift) DE 38 24 082 A1 discusses a fuel assembly including a central water rod. The fuel arrangement includes shorter fuel rods than conventional fuel rods.
Despite the advances in the art of fuel assembly designs, a need still exists for a design that provides the maximum number of fuel rods with adequate moderation that is economical and reliable.